Method for capturing carbon dioxide and nitrogen oxides in flue gas and conversion thereof to carbon source and nitrogen source needed for algae growth

This application is based upon and claims priority under 35 U.S.C. 119 from Taiwan Patent Application No. 111150464 filed on Dec. 28, 2022, which is hereby specifically incorporated herein by this reference thereto.

The present invention relates to a method for treating a gas, and particularly to a method for treating contaminant in a flue gas for reutilization.

With industrial development, the waste gas, emitted from boilers of factories and thermal power plants, etc., increases greatly, which causes severe air pollution. Among them, polluted air comprising sulfur oxides and nitrogen oxides and greenhouse gas, such as carbon dioxide, are largely produced by burning fossil fuel. Once the sulfur oxides and nitrogen oxides are emitted into the air, acid rain will be generated immediately, which will cause soil acidification and affect crop growth. As for carbon dioxide, it is one of the greenhouse gas most responsible for global warming. At present, the Environmental Protection regulation stipulates that the flue gas discharged from chimneys must go through treatment processes such as desulfurization and denitrification before discharging, so that the sulfur oxides and the nitrogen oxides can be reduced to a certain concentration. In the existing technology, a common way to remove nitrogen oxides is to reduce nitrogen oxides into harmless stable nitrogen and release it into the air. However, this method cannot make good use of the discharged waste, which is a pity.

In addition, the emitted waste gas increases the amount of greenhouse gas and strengthens the greenhouse effect, which results in global warming. Thus, the extreme climate increases, floods and droughts frequently happen, and extremely cold and hot climates normalize. These impact the crop harvest and make the ice field melt and sea level rise, seriously affecting the ecology and the environment. Therefore, the reduction of carbon dioxide, the main greenhouse gas, is the goal of the global effort.

In order to solve the problem of carbon dioxide and greenhouse effect, lots of carbon capture, storage and utilization technologies have been developed, including: converting carbon dioxide into various chemical substances, such as acetic acid or polycarbonate (PC) and other chemical plastic products. These methods seem reasonable and can achieve some economic effects from industrial recycling. However, in fact, humans do not consume so many chemical products; therefore, carbon dioxide in the atmosphere still cannot be largely removed.

Currently, the fossil fuels burned by human beings are organic substances accumulated by photosynthesis of plants since ancient times. These fossil fuels contain a large number of carbon elements and nitrogen elements which are essential to plant growth. If we only utilize the energy of fossil fuels and arbitrarily discharge these large numbers of carbon elements and nitrogen elements, it will not only destroy the natural carbon cycle, but also cause negative effects of pollution and global warming. Therefore, introducing these large numbers of carbon elements and nitrogen elements into natural carbon cycle is the fundamental solution to the pollution and global warming problems that human beings encounter presently.

In view of the fact that the existing technology is unable to reduce a large amount of carbon dioxide in the waste gas discharged after burning fossil fuels, the purpose of the present invention is to capture a large amount of carbon dioxide and nitrogen oxides in the flue gas discharged after burning fossil fuels, and also to convert the captured subjects into other products for subsequent use, such as applying these products to algae cultivation, so that the huge amount of carbon dioxide can return to the natural carbon cycle and be transferred to other layers of carbon cycle rapidly, thereby greatly reducing carbon dioxide in the waste gas, and greatly reducing acid rain generated by nitrogen oxides and its impact on the environment.

To achieve the above purpose, the present invention provides a method for treating a flue gas comprising steps as follows:

In the existing gas treatment methods, the desulfurization and denitrification technology is to decompose nitrogen oxides into nitrogen by catalysts followed by desulfurization treatment, such as wet desulfurization and dry desulfurization. Nitrogen is a stable gas and cannot be absorbed or utilized by plants, and nitrate ion is a form of nitrogen source that plants can directly absorb and utilize. According to the present invention, the flue gas can be preliminarily purified by the desulfurization step to largely reduce sulfur element, which hinders the subsequent algae growth. After that, the processes of oxygen providing and water washing are conducted, so that the nitrogen dioxide obtained by the oxidation of nitrogen oxides in the flue gas can be dissolved in water to form a nitric acid solution, and then the basic solution is contacted with the rinsed flue gas for an acid-base neutralization reaction. Therefore, the present invention can achieve the effect of greatly reducing carbon dioxide and nitrogen oxides in flue gas, reducing greenhouse gas and acid gas emissions, purifying air quality and improving ecology and the environment. Moreover, the products obtained by the treating method of the present invention can be used as carbon source and nitrogen source for subsequent algae cultivation. Consequently, the present invention not only greatly reduces greenhouse gas in waste gas, but also fully utilizes the waste in flue gas. That is, the present invention not only can protect the environment but also has commercial value.

Preferably, the nitrogen oxides may include nitrogen dioxide (NO).

According to the present invention, sulfur oxide includes, but is not limited to, sulfur dioxide and sulfur trioxide.

According to the present invention, the desulfurized flue gas includes nitric oxide, nitrogen dioxide, and carbon dioxide.

According to the present invention, the sources of the flue gas are, but not restricted to, the waste gas produced from burning coal, natural gas, or fuel oil by thermal power plants, steel mills, concrete plants, petrochemical plants, oil refineries, paper mills, heating plants, etc.

Preferably, the concentration of the sulfur oxides in the flue gas is less than 500 ppm.

Preferably, in the step (B) of the aforementioned method, the volume ratio of the oxygen to nitric oxide is from 1:1.5 to 1:2.0. More preferably, in the step (B) of the aforementioned method, the volume ratio of the oxygen to nitric oxide is from 1:1.7 to 1:1.9, such as, 1:1.8.

Preferably, in the step (B) of the aforementioned method, a catalyst may be used to enhance the oxidation reaction of oxygen reacting with the nitric oxide in the desulfurized flue gas to obtain the oxidized flue gas, wherein the catalyst may be metal or active carbon. More preferably, the metal is gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), or ruthenium (Ru), etc.

Preferably, the concentration of the nitric acid solution obtained in the step (C) of the aforementioned method is 50 weight percent (wt %) to 70 wt %. More preferably, the concentration of the nitric acid solution obtained in the step (C) of the aforementioned method is 55 wt % to 65 wt %.

Preferably, the rinsed flue gas obtained from the step (C) of the aforementioned method is continuously oxidized, so that the nitric oxide therein can be used up after several oxidation cycles.

Preferably, the aforementioned method further comprises step (E): adjusting the pH value of the nitric acid solution to 6.5 to 8 to obtain a pH-adjusted nitric acid solution. This pH range makes the pH-adjusted nitric acid solution suitable for adjusting the optimal pH range for microalgae growth in the subsequent steps, and able to provide nitrogen element needed for fast growth of microalgae.

Preferably, the aforementioned method further comprises step (F): adding the weak basic solution and the pH-adjusted nitric acid solution to a microalgae cultivation tank containing microalgae. This can provide the carbon source for microalgae in water for photosynthesis, so that the microalgae can be further processed to produce functional foods, animal feeds and the biodiesel. Besides, the microalgae cultivation solution can be recycled for carbon dioxide adsorption. Thus, the product of the present invention can be effectively used. Preferably, the microalgae cultivation tank containing microalgae contains a microalgae solution, wherein the microalgae solution comprises 0.01 wt % microalgae to 0.5 wt % microalgae. In another embodiment, the weak basic solution (i.e. carbon source) or the pH-adjusted nitric acid solution (i.e. nitrogen source) was dried before adding to the microalgae cultivation tank containing microalgae. Since the nitrogen source or carbon source can be added to the microalgae cultivation tank containing microalgae in a solid form, the drying step makes it easier for transporting the carbon source and the nitrogen source to other places for microalgae cultivation.

Preferably, the weight ratio of addition amount of the weak basic solution (i.e. carbon source) to the pH-adjusted nitric acid solution (i.e. nitrogen source) to the microalgae solution in the microalgae cultivation tank containing microalgae is to 2000:10 to 100:8000 to 20000.

6 Preferably, in the step (E) of the aforementioned method, the sodium hydroxide is added to the nitric acid solution to obtain the pH-adjusted nitric acid solution, wherein the pH-adjusted nitric acid solution is a pH-adjusted sodium nitrate solution. In order to avoid the growth environment of microalgae being too acidic, which results from that a large amount of nitric acid solution is directly added to the microalgae cultivation tank, alkaline substances can be added to the nitric acid solution to adjust the pH range, so that the microalgae can grow in a suitable environment. Adjusting pH with sodium hydroxide can promote the growth of microalgae better than other alkaline substances such as potassium hydroxide, magnesium oxide or calcium carbonate. Take cultivation of cyanobacteria as an example, 2 kilogram (kg) to 4 kg of sodium hydroxide can be added to 3 kg to 5 kg of 70 wt % nitric acid solution, followed by adding to 1 metric ton of microalgae solution, which makes the cyanobacteria have the best growth condition, that is, Cyanobacterial biomass can grow rapidly. More preferably, if 10 kg of NaHCOis added as carbon source at the same time, the total biomass of microalgae can be multiplied within 48 hours under proper water flow and sufficient lighting.

Preferably, the aforementioned step (D) includes:

Preferably, the method of desulfurizing may be dry desulfurization. By adopting the dry desulfurization, the problem of sticky wall of semi-dry desulfurization can be avoided. Specifically, sodium bicarbonate powder can be used as a desulfurization agent in the dry desulfurization. Due to the activation of sodium bicarbonate by the high temperature flue gas, microporous structures can be formed on the surface of sodium bicarbonate and can react with sulfur oxides rapidly and fully.

Preferably, the basic solution in the step (D) of the aforementioned method is a sodium hydroxide solution. One of the reasons is that the sodium bicarbonate, which is the product obtained by the reaction of sodium hydroxide and carbon dioxide, does not cause environmental pollution. Besides, it can be collected for further utilization, especially it can be used as a good carbon source and added into the microalgae cultivation tank in step (E).

Preferably, the microalgae issp.,sp.,sp.,sp.,sp.,sp.,sp.,, Cyanobacteria, or of any combination thereof.

Preferably, the step (B) is step (B′): dedusting the desulfurized flue gas and providing oxygen to react with the nitric oxide in the desulfurized flue gas for the oxidation reaction to obtain an oxidized flue gas, wherein the oxidized flue gas comprises the nitrogen dioxide and the carbon dioxide. This step can further purify the flue gas. Specifically, the dust particles in the flue gas and/or the dust particles produced by the aforementioned desulfurization process can be removed. In some embodiments, the particle size of the aforementioned dust particles is above 0.1 micrometer. Preferably, the dedusting step is to filter out the dust particles by a nanofiber tube.

Preferably, in the step (C) of the aforementioned method, the total contacting time of the basic solution with the rinsed flue gas is more than 5 seconds. More preferably, the total contacting time is more than 15 seconds. More preferably, the total contacting time is more than 1 minute.

In the step (D) of the aforementioned method, the higher the concentration of the basic solution, the easier the basic compound in the basic solution would deposit, which results in a stuck tunnel and increases the risk for the operation. However, the lower the concentration of the basic solution, the higher the needed flow rate of the basic solution. In other words, it is relatively energy-wasting. Therefore, preferably, the concentration of the basic solution is from 1 wt % to 50 wt %, but it is not limited to this. In one embodiment, the concentration of the basic solution is 3 wt % to 5 wt %, which is safe in operation and relatively energy-saving. In another embodiment, a high-concentration basic solution is adopted to generate salt crystals. For example, a sodium hydroxide having a concentration greater than 10 wt % can be adopted for spraying to generate sodium bicarbonate crystal, so that the sodium bicarbonate can be transported to other places in a solid form for algae cultivation.

Preferably, the step (D) of the aforementioned method is conducted at a temperature below 90° ° C. This temperature range can prevent the water content, comprised in the basic solution, from evaporating due to the excessively high temperature, and makes the dissolved basic compound deposit partially, resulting in a stuck tunnel for basic solution transportation easily. More preferably, the Step (D) is conducted at the temperature between 4° C. and 90° ° C., since the reaction rate of acid-base neutralization decreases at the temperature below 4° C. Additionally, considering the solubility of the salt, obtained by the reaction of said basic solution and carbon dioxide, more preferably, the step (C) is conducted at a temperature between 50° C. and 60° C.

The present invention further provides a system for treating a flue gas, the system comprising:

Preferably, the oxidation reaction unit is a ceramic fiber filter containing a catalyst. Preferably, the catalyst may be metal or active carbon. More preferably, the metal may be gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), or ruthenium (Ru).

Preferably, a nitric oxide detection valve is mounted on the rinsed flue gas outlet, wherein the nitric oxide detection valve connects to the oxidation unit through a circulating pipe and can detect the concentration of nitric oxide in the rinsed flue gas outlet, and in the case that the concentration of nitric oxide in the rinsed flue gas outlet is higher than 10 ppm, the rinsed flue gas will be reintroduced to the oxidation reaction unit.

Preferably, the nitric acid solution outlet is further in fluid communication with a pH adjustment unit. The pH adjustment unit provides sodium hydroxide for adjusting the pH value of the nitric acid to provide the optimal condition for algae growth.

The advantage of the present invention is to retain nitrogen oxides in the flue gas by means of desulfurization without denitration. Then the nitrogen dioxide is dissolved in water by the steps of oxygen treatment and water rinsing to form the nitric acid solution, and then the basic solution is contacted with the rinsed flue gas for an acid-base neutralization reaction. Therefore, the present invention can achieve the effect of greatly reducing carbon dioxide and nitrogen oxides in flue gas, reducing greenhouse gas emissions, purifying air quality and improving ecology and environment. Moreover, the products obtained by the treating method of the present invention can be used as carbon source and nitrogen source for subsequent algae cultivation. Consequently, the present invention not only greatly reduces greenhouse gases in waste gas, but also fully utilizes the waste in flue gas. That is to say, the present invention not only can protect the environment but also has commercial value.

The present invention is further explained through the following embodiments. The present invention should not be limited to the contents of the embodiments. A person having ordinary skill in the art can do some improvement or modifications which are not departing from the scope of the present invention.

First, in step (A): a flue gas was desulfurized to obtain a desulfurized flue gas. Specifically, an emitted flue gas collected from the high-temperature boilers of steelmaking comprising nitric oxide, nitrogen dioxide, sulfur oxide, carbon dioxide, and dust particles were dry-desulfurized by sodium bicarbonate to obtain a desulfurized flue gas, wherein the desulfurized flue gas comprises dust particles, nitric oxide, nitrogen dioxide and carbon dioxide.

Next, in step (B′): the desulfurized flue gas was dedusted, and oxygen was provided to react with the nitric oxide in the desulfurized flue gas for an oxidation reaction to obtain an oxidized flue gas. Specifically, the desulfurized flue gas was dedusted by passing through a ceramic fiber filter coated with a platinum metal catalyst. Then the oxygen was provided to react with the nitric oxide in the desulfurized flue gas for oxidation reaction to obtain an oxidized flue gas, wherein the oxidized flue gas comprises nitrogen dioxide and carbon dioxide. Besides, the volume ratio of oxygen provided to nitric oxide in the flue gas is 1:1.5 and the fully contacting time of gas and catalyst for reaction was 0.04 second (sec) to 1 sec.

Then, in step (C), the oxidized flue gas was rinsed with water to dissolve nitrogen dioxide in the oxidized flue gas in water to obtain a rinsed flue gas and a nitric acid solution. Specifically, the oxidized flue gas was rinsed with water spraying to dissolve nitrogen dioxide in the oxidized flue gas in water to obtain a 70 wt % nitric acid solution and a rinsed flue gas.

After that, in step (D1) the rinsed flue gas, having a flow rate of 330 m/hr, was preliminarily contacted with a 2 wt % sodium hydroxide solution, having a flow rate of 500 L/hr, in the way of co-current flow. The preliminary contact was a circulation spray lasting for 5 seconds. In the meantime, the flow rate ratio of the basic solution to the gas was 1:660. Said circulation spray refers to the rinsed flue gas sprayed by a circulated sodium hydroxide solution in a chamber, so that the carbon dioxide in the rinsed flue gas was absorbed by the sodium hydroxide solution, to obtain a preliminary basic-washed flue gas and a first sodium hydrogen carbonate solution, wherein the preliminary basic-washed flue gas comprises the remaining carbon dioxide after preliminary contact; and step (D2): the preliminary basic-washed gas, having a flow rate of 330 m/hr, was “again contacted” with the 2 wt % sodium hydroxide solution, having a flow rate of 500 L/hr, in the way of co-current flow. Besides, the term “again contacting” refers to a circulation spray lasting for 5 seconds, resulting in a second sodium hydrogen carbonate solution and a second basic-washed gas. After that, step (D2) was repeated once, in order to obtain the third sodium hydrogen carbonate solution and a basic-washed gas. The first sodium hydrogen carbonate solution, second sodium hydrogen carbonate solution and third sodium hydrogen carbonate solution, obtained from the three stages of contacting, were collected together into a storage pool, and the pH value of sodium hydrogen carbonate solution in the storage pool was measured to be 8 to 9, while the concentration of the sodium hydrogen carbonate solution in the storage pool was about 0.5 to 2 wt %. Therefore, in step (D), the rinsed flue gas was separately contacted with 2 wt % sodium hydroxide solution for three times sequentially, so that the carbon dioxide in the flue gas was mostly absorbed by the sodium hydroxide solution. Finally, a basic-washed flue gas was obtained. The content of carbon dioxide of the basic-washed flue gas was detected by a carbon dioxide detector and the result thereof showed that the content of carbon dioxide largely reduced compared to the original flue gas.

In step (D), when the sodium hydroxide solution contacted the carbon dioxide, the acid-base neutralization reacted immediately. Besides, by contacting the rinsed flue gas with the basic solution in the way of co-current flow, the contacting time of the sodium hydroxide solution with the carbon dioxide was extended. Therefore, the acid-base neutralization can fully react.

Next, in step (E): the sodium hydroxide was added to the nitric acid to obtain a pH-adjusted nitric acid solution. Specifically, the addition amount of sodium hydroxide depended on the amount of the microalgae solution in the subsequent microalgae cultivation step. In the present embodiment, 3.3 kg sodium hydroxide was added to 5.29 kg 70 wt % nitric acid solution to obtain a pH-adjusted sodium nitrate solution with a pH value from 7 to 7.5.

Finally, in step (F): the weak basic solution and the pH-adjusted sodium nitrate solution were added to a microalgae cultivation tank containing microalgae. Specifically, every 7 days, 25 kg pH-adjusted sodium nitrate solution obtained from step (E) was added to a microalgae cultivation tank containing 10 metric ton microalgae solution as nitrogen source. Meanwhile, 0.5 wt % to 2 wt % sodium bicarbonate obtained from step (D) was dried to obtain solid sodium bicarbonate and then 95 kg solid sodium bicarbonate was added to said microalgae cultivation tank containing 10 metric ton microalgae solution as carbon source every 7 days. In the present embodiment, a sodium nitrate solution with specific pH value obtained by adding specific amount of sodium hydroxide in step (E), and the specific addition amount of pH-adjusted sodium nitrate solution and sodium bicarbonate solution in step (F) can make cyanobacteria grow optimally, so that harvested microalgae powder can be maximum: 34.5 kg harvested microalgae powder every 7 days. Compared with the better addition amount of pH-adjusted sodium nitrate solution and sodium bicarbonate solution, if the addition amount of the nitrogen source and carbon source is not controlled in the optimal range, the produced microalgae powder would be less, which is only about 1/10 of the present invention, that is, 10 metric tons of algae water can only produce about 3 to 3.78 kg of microalgae powder. Generally, the weight of dried microalgae powder is about 1/3500-1/2500 of the wet weight of microalgae solution after 7 days of cultivation.

is a schematic diagram of the system for treating flue gas of the present example, wherein the double solid line indicates the pipeline where liquid or gas flows.

As shown in, the system for treating flue gasof the present invention comprises: a desulfurization part, an oxidization part, a rinsing partand a basic washing part, wherein the desulfurization partcontains a flue gas inlet, a desulfurization reaction unitand a desulfurized flue gas outlet, wherein the desulfurization reaction unitis respectively in fluid communication with the flue gas inletand the desulfurized flue gas outlet. The flue gas inletis for introducing a flue gas into the desulfurization reaction unit, wherein the flue gas comprises nitrogen oxides, such as nitric oxide and nitrogen dioxide, sulfur oxides, and carbon dioxide. The desulfurization reaction unitprovides a desulfurization agent for reacting with the sulfur oxides in the flue gas to obtain a desulfurized flue gas. The desulfurized flue gas outletis for discharging the desulfurized flue gas.

The oxidization partcontains an oxygen providing unit, an oxidation reaction unit, and an oxidized flue gas outlet, wherein the oxidation reaction unitis respectively in fluid communication with the desulfurized flue gas outlet, the oxygen providing unit, and the oxidized flue gas outlet. The oxygen providing unitprovides oxygen to the oxidation reaction unitfor oxidizing the nitric oxide in the desulfurized flue gas to obtain an oxidized flue gas, which is discharged from the oxidized flue gas outlet, wherein the oxidized flue gas comprises nitrogen dioxide and carbon dioxide

The rinsing partcontains a rinsing tower, a rinsed flue gas outlet, and a nitric acid solution outlet, wherein the rinsing toweris respectively in fluid communication with the oxidized flue gas outlet, the rinsed flue gas outletand the nitric acid solution outlet. The rinsing towerprovides water for rinsing the oxidized flue gas and dissolving the nitrogen dioxide in the oxidized flue gas in water to obtain a rinsed flue gas and a nitric acid solution. The rinsed flue gas outletis for discharging the rinsed flue gas. The nitric acid solution outletis for discharging the nitric acid solution. In other embodiments, the nitric acid solution outletis in fluid communication with a pH adjustment unit, which can adjust the pH value of the nitric acid solution.

The basic washing partcontains a basic solution spraying unit, a basic-washed flue gas outlet, and a weak basic solution outlet, wherein the basic solution spraying unitis respectively in fluid communication with the rinsed flue gas outlet, the basic-washed flue gas outlet, and a weak basic solution outlet. The basic solution spraying unitprovides a basic solution to absorb carbon dioxide in the rinsed flue gas to obtain a basic-washed flue gas and a weak basic solution. The basic-washed flue gas outletis for discharging the basic-washed flue gas, and the weak basic solution outletis for discharging the weak basic solution.

The method of utilizing the system for treating flue gas of the present invention is stated as follows:

First, an emitted flue gas collected from the high-temperature boilers of steelmaking was introduced to the desulfurization partthrough the flue gas inlet; wherein the flue gas comprised nitrogen oxides such as nitric oxide and nitrogen dioxide, sulfur oxides, carbon dioxide. The sulfur oxides in the flue gas were absorbed by the desulfurization agent: sodium bicarbonate powders in the desulfurization reaction unit, so that a desulfurized flue gas was obtained.